Olivocochlear neurons whose cell bodies reside in the superior olivary complex (SOC) provide an efferent feedback to the cochlea. This application seeks to further understand the development of auditory function in newborns by studying some of the details of the maturation of these efferent neurons and their cochlear terminations. The proposed project involves an ambitious and yet systematic approach studying both the neurochemical and morphological development of olivocochlear neurons within a single species, the golden Syrian hamster. The specific aims are to demonstrate the types of olivocochlear neurons that are present in the brainstem lateral superior olive (LSO) after birth and to demonstrate the types of olivocochlear terminations that are present in the cochlea after birth. The primary focus of this research is restricted to the lateral olivocochlear (LOC) system in which the least is known about its contribution to auditory function. A combination of immunocytochemical, retrograde labeling, electron-microscopic, and in situ hybridization techniques are employed to achieve these goals. The first series of experiments will attempt to characterize the development of neurochemical markers for olivocochlear neurons within the LSO as well as for their terminations within the cochlea. Although this necessitates the immunologic identification and quantitative comparison of several neurotransmitters and/or their synthetic enzymes that are known to be present in the LOC system, the main emphasis will be placed on the co-transmitter calcitonin gene-related peptide (CGRP), which may be uniquely identified with the LOC. The second series of experiments will attempt to characterize in the postnatal brainstem, the morphological identities of LOC cell bodies using several retrograde labeling techniques. These experiments will also provide data on the relative numbers of neurons in the LOC system between postnatal and adult brains. A third series of experiments are done to provide a systematic analysis of LOC terminals as a function of postnatal age. Using immunoelectron-microscopic methods, the main emphasis will be on correlations of CGRP immunoreactive fibers and terminals under hair cells with complementary ultrastructural data. In the final series of experiments the developmental specificity of CGRP for the LOC system is addressed In situ hybridization histochemistry will be used to determine more precisely the extent, quantity, and localization of CGRP in the postnatal hamster.